1. Field of the Invention
The present invention relates to a control system and method utilizing servo-accelerometers to determine the rotation rate and angular position information of a rotating downhole drilling tool. However, the system may be useful in any other similar apparatus where the sensors are mounted on a rotating housing and rotation rate and/or angular position information is needed.
2. Description of the Related Art
An oil or gas well often has a subsurface section that is drilled directionally towards a desired target. To reach that target, the well follows a trajectory inclined at an angle with respect to the vertical, the inclination, and oriented towards a particular compass heading, the azimuth. Although wells having deviated sections may be drilled at any desired location, a significant number of deviated wells are drilled in the marine environment. In such case, a number of deviated wells are drilled from a single offshore production platform in a manner such that the bottoms of the boreholes are distributed over a large area of a producing horizon over which the platform is typically centrally located. Wellheads for each of the wells are located on the platform structure. Directional wells may be drilled from any type of wellbore, platform or non-platform type.
A rotary steerable drilling system steers the drill bit while the drill bit is being rotated by the collar of the tool. This enables drilling personnel to readily navigate the wellbore from one subsurface oil reservoir to another. The rotary steerable drilling tool enables steering of the wellbore both from the standpoint of inclination and from the standpoint of azimuth so that two or more subsurface zones of interest can be controllably intersected by the wellbore being drilled. Rotary steerables were developed to reduce friction for extended reach situations, but also improve downhole control. Examples of rotary steerable tools are disclosed in commonly assigned U.S. Pat. Nos. 6,092,610 and 6,158,529, the entirety of which are incorporated herein by reference.
A non-rotary steerable tool has structure that provides a bend angle such that the axis below the bend point, which corresponds to the rotation axis of the bit, has a bit angle with respect to a reference, as viewed from above the tool. The bit""s angular position establishes the azimuth or compass heading at which the deviated borehole section will be drilled as the mud motor is operated. Furthermore, the bit""s angular position controls the tendency for the well to build or drop in inclination. After the bit angle has been established by slowly rotating the drill string and observing the output of various orientation devices, the mud motor and drill bit are lowered, with the drill string non-rotatable to maintain the selected bit angle, and the drilling fluid pumps, xe2x80x9cmud pumpsxe2x80x9d, are energized to develop fluid flow through the drill string and mud motor, thereby imparting rotary motion to the mud motor output shaft and the drill bit that is fixed thereto. The presence of the bend angle causes the bit to drill on a curve until a desired borehole inclination has been established. To drill a borehole section along the desired inclination and azimuth, the drill string is then rotated so that its rotation is superimposed over that of the mud motor output shaft, which causes the bend section to merely orbit around the axis of the borehole so that the drill bit drills straight ahead at whatever inclination and azimuth have been established. Measurement-while-drilling xe2x80x9cMWDxe2x80x9d systems commonly are included in the drill string above the mud motor to orient the angular position of the bent angle and monitor the progress of the borehole being drilled so that corrective measures can be instituted if the various borehole parameters indicate variance from the projected plan.
Various rotary steerable downhole drilling tools make use of a non-rotating section that contains sensors that determine the direction to apply a force or point the drill bit. In the type of these tool having a non-rotating section that houses the sensors, some of these prevent the non-rotating section from rotating by contact with the well bore. Others stabilize the non-rotating section using control from a rotating rate sensor. Accelerometer data can be filtered to remove noise from shock and vibration, and used directly to determine the direction to apply a steering force. In the type of tool where the section containing the sensors rotates with the collar, rotation rate is measured by either a gyroscope or magnetometers. Control is applied to the steering section to counteract the rotation rate to make it geostationary.
Tri-axial magnetometers (3 magnetometers mounted orthogonal to each other, 1 axial and 2 radial) are commonly used to determine rotation rate and position of the tool. The rotation rate, or angular velocity, relates to the speed of rotation of the tool during drilling. The position of the tool, often referred to as the xe2x80x9ctoolfacexe2x80x9d, relates to the steering direction of the tool with respect to vertical (the direction opposite the earth""s gravity). By manipulating the rotation rate and/or toolface, the tool may be steered in the desired direction. However, when drilling in the same direction as the earth""s magnetic field, the radial component of tri-axial magnetometers becomes too small to be used to determine rotation rate and/or tool face for steering. Gyroscopes work in any magnetic field and can measure rotation rate, but currently available gyroscopes are too inaccurate to generate position information, and do not work well at high temperatures, or during extreme shock and vibration, common to downhole environments.
There remains a need for improved steering control, particularly when drilling into the earth""s magnetic field. The present invention utilizes rotational and offset accelerometers to obtain rotation rate and toolface to meet one or more of these needs.
Briefly, a system and method are provided for determining rotation rate and angular position information of a rotating downhole drilling tool. First, second and third accelerometers are mounted to a collar that is controlled to rotate in the downhole drilling tool. Each of the first, second and third accelerometers are positioned so that their respective measurement points are centered on an axis of rotation and aligned with a corresponding x, y and z Cartesian coordinate axis of the collar, wherein the x-axis is the axis of rotation of the collar. A fourth accelerometer is mounted to the collar and positioned offset from the axis of rotation of the collar by an offset distance and aligned with the second accelerometer. The fourth accelerometer generates a signal representing centripetal acceleration of the collar as a function of the offset distance. The signals output by the accelerometers are processed to generate therefrom one or both of collar rotation rate and toolface position of a bit shaft coupled to the collar through a geostationary offset mandrel. In an alternate embodiment, the directional accelerometers may be offset with respect to the x, y and z axes.
An embodiment of the invention relates to a system for determining rotation rate and position information of a rotating downhole drilling tool. The system includes an inclinometer, an offset accelerometer, an analog to digital converter and a processor. The inclinometer is mounted to a collar in the drilling tool. The inclinometer comprising multiple accelerometers positioned so that their respective measurement points are centered on the axis of rotation and aligned with a corresponding x, y and z Cartesian coordinate axis of the collar. The inclinometer generates output signals representing position of the collar with respect to gravity. The offset accelerometer mounted to said collar and positioned offset from the axis of rotation of the collar by an offset distance and aligned with one of the accelerometers in the inclinometer. The offset accelerometer generates a signal representing centripetal acceleration of the collar as a function of the offset distance. The analog to digital converter is coupled to the inclinometer and to the offset accelerometer to convert the output signals thereof into digital signals. The processor device is coupled to the analog to digital converter to process the digital signals and generate therefrom one or both of collar rotation rate and position of a toolface of a bit shaft coupled to the collar through a geostationary offset mandrel.
Another embodiment relates to a steerable rotating downhole drilling tool. The tool includes an inclinometer mounted to a collar in the drilling tool and an offset accelerometer. The inclinometer is provided with directional accelerometer capable of taking collar measurements for determining desired drilling parameters. The offset accelerometer is mounted to said collar offset a distance from the inclinometer. The offset accelerometer capable of measuring centripetal acceleration of the collar for adjusting one or more of the collar measurements whereby more accurate desired drilling parameters may be determined.
Another embodiment relates to a method for generating rotation rate and/or toolface position information of a rotating downhole drilling tool. The method includes the steps of detecting an inclination of a rotating collar in a downhole drilling tool that drives a bit shaft to form a borehole in an earth formation using accelerometers mounted to said collar, detecting centripetal acceleration of the collar using an offset accelerometer mounted to said collar offset by a distance from the axis of rotation of the collar, and generating one or both of collar rotation rate and toolface position of a bit shaft coupled to the collar through a geostationary offset mandrel from the detected inclination of the collar and the centripetal acceleration of the collar.
Another embodiment relates to a method for steering a rotating downhole drilling tool having a drill collar. The steps include detecting acceleration of the collar using at least one directional accelerometer mounted to said collar, detecting acceleration of the collar using an offset accelerometer mounted to said collar, the offset accelerometer positioned parallel to at least one directional accelerometer a distance therefrom, measuring the resolver angle of the collar, generating collar rotation rate of a bit shaft and a toolface position, and adjusting the counter rotation speed of the offset mandrel whereby the tool is steered in the desired direction.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.